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| Science Forum Index » Medicine - Vision Forum » spherical in eyeball lens vs. spherical in glasses... |
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| Liz... |
 Posted: Wed Oct 28, 2009 8:26 pm |
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Guest
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:-)
Is "spherical aberration" in the eyeball the same thing as "spherical
correction" in a glasses prescription?
e.g.
If you have cataract surgery and an IOL implanted and are left with
(I'm making this number up) +0.30 um (I think it's um)
of spherical aberration, will that look the same to you, visually, as
if you had on a pair of glasses in which the scrip was wrong by +0.30
diopters?
It won't, right? They're different, right?
Check this website (ignore the other junk on it), where it shows a
series of "E"s. Also further down, where it shows two views of green
squares.
http://www.tecnisiol.com/optics.htm
If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?
thanks,
Liz
(easily confused by words that are the same, like "spherical" and
"spherical") - thanks.... |
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| Salmon Egg... |
| Posted: Thu Oct 29, 2009 7:42 am |
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Guest
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In article
<c892c095-020f-4de7-baf9-c7a6a13ba159 at (no spam) j19g2000yqk.googlegroups.com>,
Liz <fraternobombus at (no spam) yahoo.com> wrote:
[quote]:-)
Is "spherical aberration" in the eyeball the same thing as "spherical
correction" in a glasses prescription?
e.g.
If you have cataract surgery and an IOL implanted and are left with
(I'm making this number up) +0.30 um (I think it's um)
of spherical aberration, will that look the same to you, visually, as
if you had on a pair of glasses in which the scrip was wrong by +0.30
diopters?
It won't, right? They're different, right?
Check this website (ignore the other junk on it), where it shows a
series of "E"s. Also further down, where it shows two views of green
squares.
http://www.tecnisiol.com/optics.htm
If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?
thanks,
Liz
(easily confused by words that are the same, like "spherical" and
"spherical") - thanks....
[/quote]
As I read this post, I realized how confusion can arise.
Ideally, the eye takes a bundle of light from a point in a viewed scene
and converts it to a cone of light that converges to a point on the
retina. In terms of light waves, there is a collapsing cone of light
with a spherical wavefront centered on the retina.
Ignoring astigmatism, there are two major things that give bad vision.
The radius of the wavefront sphere is not on the retina. This can lead
to the blurred vision of myopia or hyperopia. This is optically
corrected with a lens that converts the wrong wave front radius to the
correct radius. The power of the lens required to do that is called the
spherical correction.
The problem with spherical aberration is that the wavefront does not
have the shape of a sphere (leaving out cylindrical error of optometric
astigmatism from consideration here). Such spherical wavefront
correction is not possible with a simple lens. Modern refractive surgery
makes it possible to measure aberration and actually sculpt the eye's
optical system to take most of such aberration out.
Another source of confusion is that spherical lenses, that is lenses
with perfect spherical surfaces do have spherical aberration. The
aberration refers to errors in the wavefront rather than error in the
construction if the lens. A large lens without spherical aberration will
usually not have spherical surfaces.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit. |
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| Liz... |
| Posted: Thu Oct 29, 2009 10:03 am |
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I know what spherical aberration in the eye's lens is and does; Mike
explained it in earlier posts.
I know that, with too much aberration, the light from the edges of the
lens doesn't end up focused on the retina. I think it happens mostly
with light going through the edges, not through the center. ?
"A large lens without spherical aberration will usually not have
spherical surfaces."
Right. I know.
My question comes from the fact that docs can measure your cornea's
(usually positive) S.A. and choose an aspheric IOL with a
corresponding (usually negative) S.A., in order to leave you with zero
S.A. after operating. (Or so they say; I doubt the process is ever
that accurate.)
So this made me wonder how important it is to get that S.A. correction
right with the IOL.
So I wondered what the view would look like if the S.A. were off.
That's why I looked at that web site. The blur in the pictures, they
claim, is representative of spherical aberration. (Assuming
everything else in the visual system was OK.)
These pictures don't look good.
But I thought, can't they just fix this particular problem with
glasses? So I asked:
[quote]Is "spherical aberration" in the eyeball the same thing as "spherical correction" in a glasses prescription?
No.
[/quote]
Uh oh.
In other words if you get the wrong diopter implant (which we are told
to expect some degree of inaccuracy in), then that's correctable with
glasses. But if you end up with spherical aberration in the cornea +
IOL combination, then that's not correctable with glasses.
(I am *not* talking about astigmatism.)
Then:
[quote]Modern refractive surgery makes it possible to measure aberration and actually sculpt the eye's optical system to take most of such aberration out.
[/quote]
You mean cutting into the cornea?
In other words, if the IOL + cornea combination ends up having
spherical aberration after surgery, and that turns out to be
bothersome (I don't know what to expect), then the only solution would
be LASIK or RK or similar. Right?
Liz
Indy |
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| Dan Abel... |
| Posted: Thu Oct 29, 2009 10:22 am |
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In article
<c892c095-020f-4de7-baf9-c7a6a13ba159 at (no spam) j19g2000yqk.googlegroups.com>,
Liz <fraternobombus at (no spam) yahoo.com> wrote:
[quote]Is "spherical aberration" in the eyeball the same thing as "spherical
correction" in a glasses prescription?
[/quote]
No.
[quote]If you have cataract surgery and an IOL implanted and are left with
(I'm making this number up) +0.30 um (I think it's um)
of spherical aberration, will that look the same to you, visually, as
if you had on a pair of glasses in which the scrip was wrong by +0.30
diopters?
It won't, right? They're different, right?
[/quote]
It's not that simple. If you look at some letters, and they are blurry,
it could be due to a number of reasons. The eye doctor will try
different types of correction, and see which fixes your vision. They
are different, but blur is blur.
[quote]If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?
[/quote]
People are often told to expect the possibility of glasses after
cataract surgery. And if you go into surgery with astigmatism, and get
"straight" cataract surgery (no astigmatism correction), then you can
expect to have the same astigmatism afterwards. If you come out of the
surgery with a different amount of spherical correction than you
desired, that can be fixed. Glasses and contacts should be able to fix
both of these.
[quote](easily confused by words that are the same, like "spherical" and
"spherical") - thanks....
[/quote]
Well, let's take another confusing word, with many meanings, like
"mental". What does that mean? Something to do with the mind. If
somebody has a mental aberration, that means that they have something
wrong with their mind. So mental must be a Bad Word. But wait! If
somebody has "mental acuity", that means their mind works well. So
maybe "mental" isn't a bad word? But "mental acuity" has an invisible
"good" in front of it. If somebody has "bad mental acuity", that means
they aren't too sharp.
I'm a layperson, so don't take the following too seriously. "Spherical"
means that the front of the eye is normally in the shape of a section of
a sphere. If the curvature is not correct (or the internal lens isn't
the right strength, or the distance between the cornea and the retina
isn't right), then your vision will be blurry, because the image is not
focused on the retina. Since, in the office, the eye doctor cannot
change the last two factors, spherical correction is applied in front of
the cornea, either glasses or contacts, so that the image is focused on
the retina. If your cornea is not spherical, that is called "spherical
aberration", and that also will cause blur. That cannot be fixed with
spherical correction, since you need different correction according to
the axis. This is called "astigmatism" (although astigmatism can be
caused by other things), and is easily fixed by glasses. Contacts are
trickier, but "toric" contacts will fix astigmatism, and I wore them for
many years.
--
Dan Abel
Petaluma, California USA
dabel at (no spam) sonic.net |
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| Salmon Egg... |
| Posted: Thu Oct 29, 2009 3:51 pm |
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Guest
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In article
<bc3ed608-9f0d-4252-aefd-d520e8bcc037 at (no spam) v15g2000prn.googlegroups.com>,
Liz <fraternobombus at (no spam) yahoo.com> wrote:
[quote]Then:
Modern refractive surgery makes it possible to measure aberration and
actually sculpt the eye's optical system to take most of such aberration
out.
You mean cutting into the cornea?
In other words, if the IOL + cornea combination ends up having
spherical aberration after surgery, and that turns out to be
bothersome (I don't know what to expect), then the only solution would
be LASIK or RK or similar. Right?
[/quote]
The fundamental optical process describing imaging is Fermat's
principle. For rays emanating from an object point to end up at one
image point, the optical paths (the sum of distances times refractive
indexes) for all paths have to be equal to one another. It does not
matter where along the path an adjustment is made to equalize such
paths. The Hubble telescope's primary mirror was incorrectly shaped. A
supplementary lens was fabricated to equalize these path lengths to
compensate for the path error produced by the primary mirror.
The same is true for the eye. Various path length errors get compensated
by introducing optical path length changes in various ways. The can be
the variation of thickness of a lens, the sculpting of the cornea, or a
changed shape of the crystalline lens. Optically, it does not make much
difference where or how the path correction is made.
Unfortunately, the required correction depends upon where the object
point is with respect to the eye's axis. On axis, it is possible to get
complete correction in principle. However, as you go off axis, the on
axis correction no longer works and can make things worse.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit. |
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| Dan Abel... |
| Posted: Thu Oct 29, 2009 4:39 pm |
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Guest
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In article
<bc3ed608-9f0d-4252-aefd-d520e8bcc037 at (no spam) v15g2000prn.googlegroups.com>,
Liz <fraternobombus at (no spam) yahoo.com> wrote:
[quote]Is "spherical aberration" in the eyeball the same thing as "spherical
correction" in a glasses prescription?
No.
Uh oh.
In other words if you get the wrong diopter implant (which we are told
to expect some degree of inaccuracy in), then that's correctable with
glasses. But if you end up with spherical aberration in the cornea +
IOL combination, then that's not correctable with glasses.
(I am *not* talking about astigmatism.)
[/quote]
But *I* was. Perhaps it would be best to ignore what I wrote. I don't
think we are talking about the same thing at all.
--
Dan Abel
Petaluma, California USA
dabel at (no spam) sonic.net |
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| Liz... |
| Posted: Thu Oct 29, 2009 9:06 pm |
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Guest
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[quote]If you have the surgery, and afterwards are left with some + or - spherical aberration in the eye (lens and cornea), can that aberration be corrected with glasses?
[/quote]
[quote]... Optically, you can correct the spherical aberration of one optical element by introducing another one with the same magnitude of aberration, but different direction.
[/quote]
Right. That's how those aspheric IOLs work, too. One or both
surfaces of the IOL are shaped to correct, or partly correct, the
spherical aberration in the cornea. (The S.A. in people's corneas
varies, so the degree of correction also varies in dif. aspheric
IOLs.)
[quote]It doesn't sound like the kind of mistake that's likely to be made, however.
[/quote]
It's not a mistake, exactly. It's just a result. If you implant an
IOL that has positive S.A. into an eye whose cornea also has positive
S.A., you end up with even more positive S.A., which might (???) cause
some blur or glare.
Liz
Indy
USA |
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| Liz... |
| Posted: Thu Oct 29, 2009 9:20 pm |
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[quote]If you have the surgery, and afterwards are left with some + or - spherical aberration in the eye (lens and cornea), can that aberration be corrected with glasses?
[/quote]
[quote]Yes, it could be corrected with glasses. But not standard glasses. I don't know whether this is the kind of thing a typical optometrist is able to offer or not.
[/quote]
[quote]It's the reason for "aspheric" lenses, where one or both surfaces are made
aspheric in order to reduce abberration.[/quote]
I thought those glasses were meant to be less aberrant than normal
glasses, but not meant to correct an aberration in the person's eye.
True?
(I'm still not talking about astigmatism, only symmetrical S.A. I
know nobody is ever perfectly symmetrical, but I'm only asking about
the symmetrical part.)
[quote]In routine optometry spherical abb is addressed by manufacturers who offer
lens blanks with aspheric surfaces. There's a choice between standard and
aspheric lenses, but the asphericity won't be custom-ground for any one
individual.
Contact lenses produce less spherical abb than glasses.
[/quote]
So.. is the answer to my question (at top) yes, or no?
thanks,
Liz |
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| Mike Ruskai... |
| Posted: Thu Oct 29, 2009 10:25 pm |
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Guest
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On or about Wed, 28 Oct 2009 23:26:52 -0700 (PDT) did Liz
<fraternobombus at (no spam) yahoo.com> dribble thusly:
[quote]If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?
[/quote]
Yes, it could be corrected with glasses. But not standard glasses. I don't
know off hand whether this is the kind of thing a typical optometrist is able
to offer or not. But optically, you can correct the spherical aberration of
one optical element by introducing another one with the same magnitude of
aberration, but different direction.
That's how they dealt with the error in the Hubble Space Telescope, and how
some telescope designs work fundamentally (the Schmidt camera,
Schmidt-Cassegrain telescope, Maksutov-Cassegrain, etc.).
It doesn't sound like the kind of mistake that's likely to be made, however.
--
- Mike
Ignore the Python in me to send e-mail. |
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| Mike Tyner... |
| Posted: Fri Oct 30, 2009 12:19 am |
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"Mike Ruskai" <BUTthannydI at (no spam) DONTearthlinkLIKE.netSPAM> wrote
[quote]Yes, it could be corrected with glasses. But not standard glasses. I
don't
know off hand whether this is the kind of thing a typical optometrist is
able
to offer or not. But optically, you can correct the spherical aberration
of
one optical element by introducing another one with the same magnitude of
aberration, but different direction.
[/quote]
It's the reason that most glasses are convex on the front, up to -8 or so.
It's the reason for "aspheric" lenses, where one or both surfaces are made
aspheric in order to reduce abberration.
In routine optometry spherical abb is addressed by manufacturers who offer
lens blanks with aspheric surfaces. There's a choice between standard and
aspheric lenses, but the asphericity won't be custom-ground for any one
individual. Sorta funny because the cornea itself is never spherical and the
eccentricity can be either prolate or oblate.
Contact lenses produce less spherical abb than glasses.
-MT |
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| Mike Tyner... |
| Posted: Fri Oct 30, 2009 8:37 am |
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Guest
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"Liz" <fraternobombus at (no spam) yahoo.com> wrote
[quote]I thought those glasses were meant to be less aberrant than normal
glasses, but not meant to correct an aberration in the person's eye.
True?
[/quote]
Well, yes, but SA is SA and the SA from glasses usually overwhelms the tiny
amounts generated at the cornea or internally. And when the pupils are
small, SA from the cornea and lens drops to nil, compared to glasses.
Most corneas are prolate, which offsets SA.
[quote]In routine optometry spherical abb is addressed by manufacturers who offer
lens blanks with aspheric surfaces. There's a choice between standard and
aspheric lenses, but the asphericity won't be custom-ground for any one
individual.
So.. is the answer to my question (at top) yes, or no?
[/quote]
You're asking whether glasses can be sculpted to create a perfect wavefront,
like topography-driven lasers sculpt the eye? Yes, but you have to look
through the exact center of the lens from then on. :)
Kidding aside, I haven't seen any system for designing individual spectacles
to minimize internal SA. There may well be. But internal SA is tiny compared
to the SA created by glasses, and almost nonexistent in bright light.
-MT |
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| Salmon Egg... |
| Posted: Fri Oct 30, 2009 9:24 am |
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Guest
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In article <96qke5p4eqs1hpe72l076g9347gns44dt3 at (no spam) 4ax.com>,
Mike Ruskai <BUTthannydI at (no spam) DONTearthlinkLIKE.netSPAM> wrote:
[quote]On or about Wed, 28 Oct 2009 23:26:52 -0700 (PDT) did Liz
fraternobombus at (no spam) yahoo.com> dribble thusly:
If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?
Yes, it could be corrected with glasses. But not standard glasses. I don't
know off hand whether this is the kind of thing a typical optometrist is able
to offer or not. But optically, you can correct the spherical aberration of
one optical element by introducing another one with the same magnitude of
aberration, but different direction.
That's how they dealt with the error in the Hubble Space Telescope, and how
some telescope designs work fundamentally (the Schmidt camera,
Schmidt-Cassegrain telescope, Maksutov-Cassegrain, etc.).
It doesn't sound like the kind of mistake that's likely to be made, however.
[/quote]
It is not that simple. The Hubble telescope is pointed in the direction
toward the object to be observed. In reading, for example, you swivel
your eyes to scan a line. You do not ordinarily turn you head to scan a
line. This means that you look through different portions of any
corrective lenses you wear while reading. What may correct for on-axis
reading is likely to introduce optical error when you swivel your eye to
look through a different portion of any corrective lens.
With refractive surgery, the cornea is sculpted to minimize total
optical error on-axis. As you scan a line of text, the correction in
your eyeball turns with the eye. If you had glasses that turned with
your eyes, then aspheric correction could end up being effective no
matter which diriction you look.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit. |
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| Mike Ruskai... |
| Posted: Fri Oct 30, 2009 5:16 pm |
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Guest
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On or about Fri, 30 Oct 2009 08:24:20 -0700 did Salmon Egg
<SalmonEgg at (no spam) sbcglobal.net> dribble thusly:
[quote]In article <96qke5p4eqs1hpe72l076g9347gns44dt3 at (no spam) 4ax.com>,
Mike Ruskai <BUTthannydI at (no spam) DONTearthlinkLIKE.netSPAM> wrote:
On or about Wed, 28 Oct 2009 23:26:52 -0700 (PDT) did Liz
fraternobombus at (no spam) yahoo.com> dribble thusly:
If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?
Yes, it could be corrected with glasses. But not standard glasses. I don't
know off hand whether this is the kind of thing a typical optometrist is able
to offer or not. But optically, you can correct the spherical aberration of
one optical element by introducing another one with the same magnitude of
aberration, but different direction.
That's how they dealt with the error in the Hubble Space Telescope, and how
some telescope designs work fundamentally (the Schmidt camera,
Schmidt-Cassegrain telescope, Maksutov-Cassegrain, etc.).
It doesn't sound like the kind of mistake that's likely to be made, however.
It is not that simple. The Hubble telescope is pointed in the direction
toward the object to be observed. In reading, for example, you swivel
your eyes to scan a line. You do not ordinarily turn you head to scan a
line. This means that you look through different portions of any
corrective lenses you wear while reading. What may correct for on-axis
reading is likely to introduce optical error when you swivel your eye to
look through a different portion of any corrective lens.
[/quote]
That's true, but you're overestimating the magnitude of the complication.
Consider a tilted component telescope, which has much higher angles than the
small scanning movements of a reading eye. The image is still usable, and the
spot diagram is much smaller than it would be for straight SA.
[quote]With refractive surgery, the cornea is sculpted to minimize total
optical error on-axis. As you scan a line of text, the correction in
your eyeball turns with the eye. If you had glasses that turned with
your eyes, then aspheric correction could end up being effective no
matter which diriction you look.
[/quote]
Or contacts. But I still contend that the angles are too small to make such a
hypothetical corrective lens impractical.
In reality, I expect the kind of error suggested would be corrected with
further surgery, if it ever occurred at all. If what you say is accurate,
they'd have to measure SA as normal procedure, which makes these speculative
lenses unlikely to ever be needed.
--
- Mike
Ignore the Python in me to send e-mail. |
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| Salmon Egg... |
| Posted: Fri Oct 30, 2009 8:42 pm |
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Guest
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In article <4dsme5lgc5c7njfesaonjnv8meaptnjsgk at (no spam) 4ax.com>,
Mike Ruskai <BUTthannydI at (no spam) DONTearthlinkLIKE.netSPAM> wrote:
[quote]It is not that simple. The Hubble telescope is pointed in the direction
toward the object to be observed. In reading, for example, you swivel
your eyes to scan a line. You do not ordinarily turn you head to scan a
line. This means that you look through different portions of any
corrective lenses you wear while reading. What may correct for on-axis
reading is likely to introduce optical error when you swivel your eye to
look through a different portion of any corrective lens.
That's true, but you're overestimating the magnitude of the complication.
Consider a tilted component telescope, which has much higher angles than the
small scanning movements of a reading eye. The image is still usable, and the
spot diagram is much smaller than it would be for straight SA.
With refractive surgery, the cornea is sculpted to minimize total
optical error on-axis. As you scan a line of text, the correction in
your eyeball turns with the eye. If you had glasses that turned with
your eyes, then aspheric correction could end up being effective no
matter which diriction you look.
Or contacts. But I still contend that the angles are too small to make such a
hypothetical corrective lens impractical.
In reality, I expect the kind of error suggested would be corrected with
further surgery, if it ever occurred at all. If what you say is accurate,
they'd have to measure SA as normal procedure, which makes these speculative
lenses unlikely to ever be needed.
--
I have pointed out before that I am not a health professional but that[/quote]
any medical treatment cannot violate the laws of physics.
I have never been a big fan of spherical aberration correction in
spectacles. I can picture aberration correction being required for
something like keratoconus or other gross distortions of the eye's
optical system.
To some people even too much treatment is not enough.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit. |
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| Salmon Egg... |
| Posted: Sat Oct 31, 2009 5:27 am |
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Guest
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In article <4dsme5lgc5c7njfesaonjnv8meaptnjsgk at (no spam) 4ax.com>,
Mike Ruskai <BUTthannydI at (no spam) DONTearthlinkLIKE.netSPAM> wrote:
[quote]Or contacts. But I still contend that the angles are too small to make such a
hypothetical corrective lens impractical.
In reality, I expect the kind of error suggested would be corrected with
further surgery, if it ever occurred at all. If what you say is accurate,
they'd have to measure SA as normal procedure, which makes these speculative
lenses unlikely to ever be needed.
[/quote]
I do not see any significant advantage to be obtained by emphasis on
spherical aberration compared to other errors. The way I understand
modern refractive surgery, it will correct for any kind of on-axis
aberration including inhomogeneity of the eye's optical media. The
caveat is that the error varies relatively smoothly with position, The
restriction to only on-axis correction is because that is the way an eye
is used. I cannot think of any technical reason that would prevent
correction for off-axis error.
The corresponding process in making optical telescopes is called
"figuring." Classical figuring is used for large primary mirrors. To get
best focussing over a small field of view, the mirror's surface was
changed from spherical to paraboloidal. Note that it is much easier to
fabricate high quality spherical surfaces than aspherical ones. So after
getting a spherical surface, the mirror would be hand polished a bit to
convert the sphere to a paraboloid. Done perfectly, it would get rid of
spherical aberration completely. The problem is that off-axis.
performance dropped off.
For large field applications, compromises between field-of-view had to
be made. That is how the Hubble telescope got into trouble. Figuring of
the mirror was incorrect for magtching thes rest of the optical train.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit. |
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